In the domain of aeronautics inerting systems are known for the generation of an inert gas such as nitrogen or any other inert gas such as carbon dioxide, and for introducing said inert gas into fuel tanks for safety reasons, that is, to reduce the risk of explosion from said tanks.
A conventional, prior art inerting system typically includes an on board inert gas generating system (OBIGGS) supplied with compressed air, for example, with compressed air diverted from at least one engine from a so-called intermediate pressure stage and/or a so-called high-pressure stage based on a flight situation. It should be noted that the use of compressed air from an engine is advantageous because it has a relatively high pressure and temperature so the air can be adjusted to a wide range of desired pressure and temperature settings. The OBIGGS is connected to the aircraft fuel tank and separates oxygen from the air.
The OBIGGS is composed of at least one air separating module containing, for example, permeable membranes such as polymer membranes passed over by an air flow. Due to the different permeabilities of the membrane to nitrogen and oxygen, the system splits the air flow so that an air flow with high nitrogen content and an air flow with high oxygen content are obtained. The air fraction enriched with nitrogen, considered to be the inert gas, is routed into fuel tanks so that the oxygen level present within the free volume of the tank is decreased. The devices required for this process such as compressors, filters, and air or water cooling modules or similar are integrated into the inerting system.
When the oxygen ratio in the empty part of the tank is below the ignition limit defined in accordance with the Federal Aviation Administration (FAA) requirements detailed in AC25,981-2A dated Sep. 19, 2008 entitled “FUEL TANK FLAMMABILITY REDUCTION MEANS” and its appendices, or pursuant to the requirements of the “European Aviation Safety Agency” (EASA), detailed in document AMC25,981, the ignition and deflagration risks are very low or even nonexistent. From the foregoing, inerting a fuel tank is composed of injecting an inert gas into the tank in order to maintain the level of oxygen present within said tank below a certain threshold, for example 12%.
An inerting system is known that is produced according to the dimensioning rules mandated, for example, by document AC25,981-2A or document AMC25,981. The flow rate of the inert gas to be injected is therefore determined at regular intervals as a function of the parameter values of a certified standard mission profile. The certified standard mission profile corresponds to the mission profile most frequently adopted by the aircraft. For example, these parameters may be a free volume of the fuel tank, and/or the rate of descent and/or climb and/or the altitude of the aircraft. The certified standard mission profile recommends, for parameter values at any given instant, the injection of a certain flow rate of inert gas, comprising a certain oxygen concentration, in order to satisfy the regulations in force.
Also known is an inerting system designed to inject inert gas into at least one fuel tank with a flow rate that is designed to meet a requirement that is determined in real time during the flight of the aircraft. This type of inerting system is not based on a certified type of mission profile imposed by the American certification authority which is often more restrictive than the actual mission carried out by the aircraft, and that therefore consumes more air. The injection of inert gas responds to an optimized inerting strategy, based for example upon an estimate of the amount of air entering the fuel tank as a function of the venting thereof, and of the actual volume of fuel consumed.
This type of known inerting system, adapted to the actual inert gas flow rate requirement, implements a function for regulating the inert gas flow rate, downstream of the air separation module. This flow control function implements a flow rate control valve, a set of sensors or a flow meter, and a computer containing a closed loop type flow rate control law. The inert gas flow control setpoint is determined from data relating to the aircraft, such as the variation in the external atmospheric pressure and/or the rate of change of altitude of the aircraft and/or the volume of fuel present within the tank and/or the fuel mass consumed by the engine.
Thus, in optimizing, and in particular in reducing the flow of inert gas injected into the fuel tank, the amount of air consumed by the air separation module decreases, which makes it possible to reduce the operating costs of the inerting system.
However, with the air entering the air separation module at a constant pressure and temperature, and at a constant atmospheric pressure, reducing the inert gas flow rate impacts upon the purity thereof. Indeed, if the flow rate of the inert gas decreases, the purity of the gas improves, i.e., the oxygen concentration thereof decreases.
It follows that this type of inerting system is oversized in relation to the actual inert gas purity requirement, and generates and injects an inert gas with a higher quality than necessary, indirectly causing excess fuel consumption for the aircraft and high operating costs